This invention relates to a process of forming an electrode for a semiconductor device, and more particularly to a process of forming an electrode of a nickel-palladium alloy for use, for example, in a Schottky barrier in a semiconductor substrate or a non-rectifying semiconductor device.
It is well known that the Schottky barrier is formed on those portions of any semiconductive material contacted by a mating metal. There have been already proposed a variety of semiconductor devices utilizing the electric characteristics of the Schottky barrier.
It is desirable that those semiconductor devices meet the following requirements:
(a) The Schottky barrier be thermally stabilized. and
(b) In order to increase the industrial value, the manufacturing process be simple and capable of mass production while being capable of inexpensively producing the semiconductor device.
In conventional semiconductor devices it has been difficult to provide a Schottky barrier meeting those requirements which will be subsequently described in detail.
Heretofore, the electrode material for the Schottky barrier has comprised principally metals having relatively excellent adhesion to semiconductive materials. Examples of such an electrode material are high melting point-metals such as molybdenum, tungsten, titanium, etc., metals of the platinum group such as platinum, rhodium, palladium etc., and further zirconium, chromium, nickel etc. In order to form the Schottky electrode of any one of those metals, it has been previously the common practice to use a sputtering or evaporation technique to form a film over the entire area of one face of a semiconductor substrate involved and then to use photolithographic and etching techniques to etch away the film on the substrate except for a predetermined portion or portions thereof. The predetermined portion or portions of the film thus left forms or form an electrode or electrodes for the Schottky barrier.
By the process as above described it has been possible to form on semiconductor substrates the electrode for the Schottky barrier which has excellent adhesion to the substrate by properly selecting the conditions for forming the film and has been particularly advantageous in that Schottky barriers formed of high melting point-metals and metals of the platinum group have excellent thermal stability. However the process of forming Schottky electrodes as above described has been disadvantageous in that the apparatus for carrying out the process is large in size and therefore expensive because of the necessity of using evacuating means. To provide a high vacuum consumes time and is detrimental to mass production. Also since the film formed on semiconductor substrate is required to be further processed according to photolithographic and etching techniques, the manufacturing cost has been additionally increased.
In order to eliminate the disadvantages of the process as above described, there have already been proposed processes of forming metals for the Schottky barrier through the utilization of an electroplating technique. Such electroplating processes have been carried out only by using a source of electric power and an electroplating tank and are therefore characterized in that they are excellent for mass production and have a high practical value. Further the necessity of using the photolithographic and etching steps has been eliminated because the barrier metal can be electrically plated on a selected portion or portions of any semiconductor substrate. However such electroplating processes have been disadvantageous in that a limitation as to the type of barrier metal exists and it is difficult to obtain barrier metals which are well adhered to associated semiconductor substrates and are thermally stable. Those barrier metals presently reported to be useful are such metals as gold and nickel. In addition, a Schottky barrier formed of either of those metals by electroplating processes has only had a heat resistance to about 200.degree. C. which has brought its usefulness into question.
Moreover, conventional electroplating techniques have not been practical because the Schottky barrier formed by these techniques was contaminated with oxide films diposed at the interface between the Schottky metal and the semiconductor or the impurities externally invaded the interface.